The present invention relates to an image processing device capable of endoscopic imaging making use of the characteristics of both of a normal light observation image obtained with white light and a special light observation image obtained with specific narrowband light.
In recent years, use is made of an endoscope system capable of so-called special light observation in which a mucosal tissue of a living body is irradiated with specific narrow wavelength band light (narrowband light) as observation light to obtain tissue information at a desired depth of the tissue of the living body.
The special light observation can simply visualize biological information which is not obtainable from the image observed with normal light (white light), such as the microstructure in the superficial layer of a neovascular vessel formed in a mucosal layer or a submucosal layer, or an enhanced lesion. For example when a cancer lesion is to be observed, a mucosal tissue is irradiated with blue (B) narrowband light suitable to the observation of the tissue in the superficial layer and green (G) narrowband light suitable to the observation of the tissue in the middle and deep layers, whereby the state of the microvessels and microstructure in the superficial layer of the tissue can be more precisely observed and the lesion can be therefore more correctly diagnosed.
The endoscope system as described in JP 3559755 B is known as an endoscope system having the functions of normal light observation and special light observation.
The endoscope system uses a light source device which includes a light source for emitting white light and a rotary filter having an R filter for converting white light into red (R) light, a G filter for converting white light into green (G) light and a B filter for converting white light into blue (B) light, and an endoscope which captures images with an imaging device for measuring without separation of incident light. The endoscope system captures images in a so-called frame sequential manner.
The endoscope system described in JP 3559755 B uses the rotary filter of a double structure including a first set of outside filters and a second set of inside filters.
The first set of outside filters are filters for use in the normal light observation which have such spectral characteristics that the wavelength ranges of the respective colors overlap each other. On the other hand, the second set of inside filters are filters for use in the special light observation which have such discrete narrowband spectral characteristics that the wavelength ranges of the respective colors are separated from each other.
Therefore, this endoscope system is capable of both of the normal light observation and the special light observation by shifting the rotational axis of the rotary filter so that the first set of filters may act on the optical path when the normal light observation is performed and the second set of filters may act on the optical path when the special light observation is performed.
However, switching between the normal light observation and the special light observation in this endoscope system requires changeover of the rotary filter. Therefore, both of a normal light observation image and a special light observation image cannot be simultaneously observed in the same biological tissue.
Under the circumstances, various proposals have been made to enable a normal light observation and a special light observation to be performed simultaneously.
For example, JP 2004-321244 A describes an endoscope system (electronic endoscope system) which involves alternately capturing and storing normal light images and special light images at predetermined timings, reading out images at predetermined timings and subjecting the images to mutually different processing to acquire a normal light observation image and a special light observation image (substantially) simultaneously.
In this endoscope system, the normal light observation image and the special light observation image are observed simultaneously by displaying the two images acquired at the same period one by one, side by side on one screen or in an overlapping manner (composition of two images).
JP 2008-43604 A describes an endoscope system which processes a color image having R, G and B images captured in limited bands with a low-pass filter or a band-pass filter to generate image signals of a normal light observation image and those of a special light observation image thereby acquiring the normal light observation image and the special light observation image simultaneously.
In this endoscope system as well, the thus acquired normal light observation image and special light observation image are observed simultaneously by displaying the two images one by one or side by side on one screen.